In an alkaline battery, a separator is used to separate a positive electrode and a negative electrode from each other to prevent a short circuit therebetween, and further, to hold an electrolyte thereon and provide a smooth electromotive reaction.
Recently, in electronic equipment, the space allotted for the alkaline battery has been reduced due to the need for miniaturization and weight-saving. Nevertheless, the performance requirement for such a smaller alkaline battery is the same as or higher than that for a conventional battery, and therefore, it is necessary to increase the capacity of the battery, and also increase the amounts of active materials in the electrodes. Thus, the volume allotted in the battery for the separator must be reduced, the thickness of the separator must be reduced thinner , and electrodes and separators must be placed in close contact to prevent the formation of a space therebetween when an electrodes-group is produced. Nevertheless, when thinner separators are used, and a strong tension is applied to ensure the close contact between the electrodes and separators, any flash occurring at the electrode can puncture the separator and cause a short circuit, an edge of the electrode might then tear the separator, and thus lower the yield.
Japanese Unexamined Patent Publication (Kokai) No. 7-29561 or Japanese Unexamined Patent Publication (Kokai) No. 8-138645, for example, disclose that fine fibers having a small diameter, for example, approximately 5 μm, are used to make the separator thinner. Nevertheless, the separator obtained from such fine fibers lacks stiffness and is easily wrinkled when an electrodes-group is produced. Because the separator has a high density, it is difficult to diffuse a poured electrolyte into the separator, and thus, the handling characteristics are poor when an electrodes-group is produced.
The inventors of the present invention made an intensive investigation of an alkaline battery separator which can remedy the above disadvantages of prior art, and can be manufactured in a good yield with a good workability. As a result, the inventors found that when a separator mainly comprising hydrophilicity-imparted, relatively thick and stiff polyolefin fibers having a fiber diameter of 8 μm or more wherein a part of the hydrophilicity-imparted polyolefin fibers is of high-strength fibers having a tensile strength of 5 g/d or more is used, a short circuit caused by a puncturing of the separator by a flash occurring at the electrode is avoided, and the separator is not torn by an edge of an electrode. Further, a battery can be stably produced in a good yield with a good workability, while the separator has the desired stiffness and is not wrinkled. Further, the present inventors also found that the separator mainly comprising hydrophilicity-imparted, relatively thick and stiff polyolefin fibers having a fiber diameter of 8 μm or more can provide spaces sufficient for holding an electrolyte, and the property required to diffuse a poured electrolyte is excellent.
Accordingly, the object of the present invention is to provide an alkaline battery separator which can be produced in a good yield with a good workability, and a process for producing the same.